Fix right-arrow navigation not entering folders under fragmented terminal input

[ScottMorris]

Summary

• Terminal input can deliver a multi-byte escape sequence (e.g. the right arrow key's \x1b[C) split across separate reads under real-world conditions such as SSH/tmux/mosh latency
• Ink's keypress parser has no way to know more bytes are coming, so it parses a lone leading \x1b byte as a standalone Escape keypress, which triggers smdu's quit-on-escape binding before the rest of the sequence arrives — from the user's side this looks like the arrow key doing nothing (or the app exiting) instead of navigating into the selected folder
• Adds StdinEscapeBuffer, a small stream wrapper between process.stdin and Ink, that holds a lone leading escape byte for 50ms to see whether the rest of the sequence follows, reassembling it if so; a genuine standalone Escape press still quits, just ~50ms later
• Root cause was confirmed by reproduction: driving the built CLI through a real pty with an artificially fragmented right-arrow sequence reliably killed the app before the fix, and navigated correctly after

Closes #92

Test plan

• pnpm build
• pnpm test (26 suites, 103 passed / 2 pre-existing skips) — includes new tests/stdinEscapeBuffer.test.ts covering reassembly, timeout-based flush, flush-on-rawmode-disable, and stream proxying
• pnpm lint
• Manually reproduced the original bug via a real pty with a split \x1b[C write — confirmed the app exited before the fix
• Re-ran the same repro with the fix applied and a realistic ~5ms fragmentation gap — navigation now works correctly, no crash
• Confirmed a genuine standalone Escape keypress still quits the app

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